1. Field of the Invention
The present invention relates to a compressor system, which includes a variable displacement compressor apparatus, and a vehicle air conditioning system having the compressor system.
2. Description of Related Art
In a prior art air conditioning system (vapor compression refrigeration system), the refrigerant pressure in a low pressure side of the air conditioning system is changed several times by intermittently disconnecting and connecting an electromagnetic clutch to return refrigerant oil, which is accumulated in a refrigerant pipe, to a fixed displacement compressor of the air conditioning system when a predetermined time period is elapsed since the time of starting the fixed displacement compressor. Such an air conditioning system is disclosed in, for example, Japanese Unexamined Patent Publication No. 2000-283576, which corresponds to U.S. Pat. No. 6,266,967.
In a case of a previously proposed variable displacement compressor apparatus of a swash plate type, the refrigerant pressure in a control pressure chamber, i.e., in a swash plate chamber is controlled to control a stroke of a piston, which compresses refrigerant, through a swash plate received in the swash plate chamber. When the refrigerant pressure in the swash plate chamber is changed, it causes a change in the balance between the force, which is applied to the piston by the refrigerant pressure in the swash plate chamber via the swash plate, and the compressive reaction force, which is applied to the piston from the compressed refrigerant. The change in the balance then causes a change in a rotational angular momentum for tilting the swash plate to change the stroke of the piston, thereby changing the displacement of the compressor apparatus.
Here, the displacement is a theoretical geometrical displacement of refrigerant discharged from the compressor apparatus per rotation of a shaft of the compressor, which is driven by, for example, a vehicle engine.
Normally, the swash plate chamber is always communicated with a refrigerant inlet side of the compressor apparatus through a choke means, such as an orifice, and is also communicated with a refrigerant outlet side of the compressor apparatus through a control valve, which can change a size of opening, i.e., a degree of opening in a passage that communicates between the swash plate chamber and the refrigerant outlet of the compressor apparatus. The pressure in the swash plate chamber is controlled by controlling a degree of opening of the control valve.
Normally, at the time of maximizing the displacement of the compressor apparatus, the control valve is closed to reduce the pressure of the swash plate chamber to a level that is substantially equal to the intake pressure of the compressor apparatus at the refrigerant inlet of the compressor apparatus. On the other hand, at the time of reducing the displacement, the control valve is opened to increase the pressure of the swash plate chamber.
Thus, when the displacement is maximized, the pressure difference between the pressure in the refrigerant inlet side of the compressor apparatus and the pressure in the swash plate chamber is significantly reduced, so that the refrigerant, which includes refrigerant oil serving as blowby gas flown into the swash plate chamber through a space between the piston and a cylinder bore, is accumulated in the swash plate chamber. Thus, the amount of refrigerant oil, which is returned to the compressor apparatus after circulating through the vapor compression refrigeration system is disadvantageously reduced.
Furthermore, the sliding surfaces of the piston and of the cylinder bore are lubricated by the refrigerant oil returned to the compressor apparatus. Thus, when the amount of refrigerant oil returned to the compressor apparatus is reduced, the piston and the cylinder bore can be seized due to the shortage of the refrigerant oil.
It is possible to increase the amount of refrigerant oil mixed into the refrigerant to address the above disadvantage. However, when the amount (i.e., an oil rate) of refrigerant oil contained in the refrigerant is increased, a heat transfer coefficient is reduced by the refrigerant oil adhered to the inner surfaces of the evaporator or of the condenser to reduce heat exchange performance.
The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to provide a novel compressor system, which is different from previously proposed compressor systems. It is another objective of the present invention to provide a vehicle air conditioning system having such a compressor system. It is a further objective of the present invention to limit accumulation of a relatively large amount of refrigerant oil in a control pressure chamber of a compressor apparatus of the compressor system.
To achieve the objectives of the present invention, there is provided a compressor system that includes a variable displacement compressor apparatus, a pressure regulating means and a control means. The compressor apparatus includes a refrigerant inlet, a refrigerant outlet, a piston and a control pressure chamber. The refrigerant is suctioned into the compressor apparatus through the refrigerant inlet. The refrigerant is discharged from the compressor apparatus through the refrigerant outlet. The piston is reciprocably driven upon rotation of the compressor apparatus to compress refrigerant supplied from the refrigerant inlet. The control pressure chamber is connected to the refrigerant inlet and the refrigerant outlet and receives refrigerant pressure from at least one of the refrigerant inlet and the refrigerant outlet. The refrigerant pressure in the control pressure chamber regulates a stroke of the piston to regulate a displacement of the compressor apparatus. The pressure regulating means is for regulating the refrigerant pressure in the control pressure chamber. The control means is for controlling the pressure regulating means. When the compressor apparatus is operated at a maximum displacement for a predetermined time period, the control means controls the pressure regulating means to gradually change the refrigerant pressure in the control pressure chamber in a manner that gradually reduces the displacement of the compressor apparatus.
To achieve the objectives of the present invention, there is also provided a vehicle air conditioning system that includes the above-described compressor system and at least one evaporator, which exchanges heat between depressurized refrigerant and air to be discharged into a passenger compartment of a vehicle.